Formally the furnace of a boiler covers the largest capacity structurally of boiler and controls the quality and the cost of the boiler greatly, and so miniaturization of the furnace of a boiler has been desired.
FIG. 10 shows a diagrammatic representation of a sectional view of a conventional watertube boiler.
In FIG. 10, (1) designates a furnace, (2) designates a secondary super heater, (3) designates a reheater and (4) designates a watertube boiler. The furnace (1) covers about 10% of a boiler as a heating surface which is not so large, but the occupied volume itself covers about 60% of the boiler.
This fact is due to the small heat liberation rate in a furnace, for example the value of heat liberation rate is only to the extent of about 100,000 Kcal/m.sup.3 H, even in a large scale capacity boiler for power generation and industry, etc. The reason for this is due to the fact that in such a boiler as a conventional boiler in which the water-wall tubes surround the large combustion flame, the heat absorption rate of the heating surface becomes larger of its own accord in proportion to the heat liberation rate in the furnace and the water tubes of a boiler are finally burnt out which brings about the so called "burn-out phenomenon".
This burn-out phenomenon is due to the fact that the heat liberation rate in the furnace of a boiler should be small in order to maintain a suitable heat absorption rate of the heating surface of a boiler, because the water-wall heating surface of a boiler is proportional to the 2nd power of its dimension against the increase of the volume of a boiler in proportion to the 3rd power of its dimension from the point of the similarity of combustion and conduction of heating according to the capacity of a boiler.
Therefore, a large space is necessary for the furnace of a large capacity boiler for power generation and industry, etc. and so accordingly the boiler has become large sized.
FIG. 11 shows a diagrammatic representation of a furnace of a conventional watertube boiler. In FIG. 11, (1) designates a furnace, (5a) designates a water-wall tube of the furnace. FIG. 12 illustrates the distribution of a heat flux of water-wall tube in the furnace of a conventional watertube boiler.
As shown in FIG. 12, water-wall tubes (5a) are given a radiation heat transfer (QoKcal/m.sup.2 H) from the combustion flame, which is a characteristic of water-wall tubes of a furnace of a conventional watertube boiler.
This radiation heat transfer is only given from the hemisphere side (7) of the furnace, but not from the hemisphere (8) of the wall side of the furnace, i.e. the hemisphere of wall side (8) of a furnace does not contribute to the heat transfer.
There is a distribution of the value of heat flux on the hemisphere side (7) of the furnace as shown by the arrows in FIG. 12. In that case, it is necessary to make the maximum value of heat flux below the critical heat flux so as not to cause a burn-out phenomenon and so there were points to be considered in design that the sum of local heat absorption rate at the circumference of a watertube of a conventional furnace should be very low. Formally, there were plans to raise the critical heat flux in order to solve the above mentioned points. For example, inner grooved water tubes were tested for use but did not succeed to raise remarkably the heat liberation rate so as to obtain a noticeable effect in the furnace.
On the other side, when the heat liberation rate in the furnace is raised, it has a defect of causing pollution because hot spots are generated in the central part of the conventional furnace and large amount of a nitrogen oxide (NOX) are exhausted in such a condition as is existed as the lumped flame in the conventional furnace. In order to suppress the critical heat flux and the amount of NOX, the furnace of a boiler cannot be made small if it is within a conventional boiler.
And, in order to exceed the limit of a boiler heretofor in use, it is necessary to adopt such a novel watertube boiler as the one in which the critical heat flux is exceedingly high, and which enables to produce high intensity combustion and to produce the low amount of NOX under high intensity combustion.